When setting up a surveying instrument such as a total station, it is necessary to determine the surveying instrument's orientation. Generally, the orientation of a surveying instrument corresponds to the deviation of the surveying instrument's alignment (direction zero of the angle measurement means) from the north direction (heading), that is—according to the common geodetic convention—from an x-axis of a horizontal coordinate system. To determine the orientation of the surveying instrument, an angle from the surveying instrument to at least one comparatively far distanced reference object having known coordinate data is measured. In general, the distance of such a reference object has to be at least 1 km.
Examples for such reference objects are a church tower cross, a cross on the summit of a mountain (summit cross), a high chimney or other distinctive objects being visibly over a long distance. Coordinate data of these reference objects in general are provided by public authorities e.g. the “Bundesamt fur Landestopographie swisstopo” in Switzerland.
A prior art method for determining a surveying instrument's orientation is described on the basis of FIGS. 1 and 2. In order to determine the surveying instrument's orientation, an angle θ between the surveying instrument's direction zero and the x-axis of the coordinate system (usually corresponding to the north direction) is determined. That is, the orientation of a surveying instrument in general is defined as the angle between the x-axis (north direction) and the surveying instrument's direction zero.
If the coordinate data of the setup point of the surveying instrument A and of the reference object B are known, from the coordinate data an oriented direction angle r0 may be calculated. The orientation direction angle is the angle between the x-axis and the line S connecting the position of the surveying instrument and the reference object B. After setting up the surveying instrument, the direction angle r to the reference object, that is, the angle between the direction zero of the surveying instrument and the line connecting the position of the surveying instrument and the reference object B is measured and the orientation angle O is determined as:O=r0−r 
In use of the surveying instrument in the field, it may happen that the surveying instrument's external orientation changes slightly for several reasons, e.g. because one foot of the tripod sinks into plaint ground, because of an inadvertent push or hit onto the surveying instrument or similar. Therefore, the surveying instrument's external orientation may be verified during a measurement process a while after starting the measurement tasks, i.e. in a proceeded or later state of the measurement process. Such a verification of the orientation may ensure that the surveying instrument didn't change and still is exactly in the initially determined orientation or, in case of a change in the orientation, may indicate that its external orientation has to be newly determined.
For verifying the surveying instrument's orientation, after a while, the direction angle r2 from the surveying instrument to the reference object B is again determined, and then the new reference angle is compared with the originally determined direction angle r1.
                              Δ          ⁢                                          ⁢          O                =                ⁢                              O            2                    -                      O            1                                                  =                ⁢                              (                                          r                0                            -                              r                2                                      )                    -                      (                                          r                0                            -                              r                1                                      )                                                  =                ⁢                              r            1                    -                      r            2                              
If the change of the orientation ΔO exceeds a predetermined value, the surveying instrument's orientation is to be newly determined. Therein, it may be remarked that the change of the orientation ΔO can be determined without knowing the absolute external orientation values O2 and O1. However, according to methods of the state of the art, such a verification method has to be carried out completely manually and is, therefore, cumbersome. Also, the reliability depends on the skill of the operating person.
Exemplarily, WO 2009/106144 A1 roughly describes a verifying procedure for checking if the originally determined orientation of a surveying instrument did or did not change during a measurement session in the field, see page 25, lines 25-33 of WO 2009/106144 A1:
“The orientation of the instrument with respect to a known direction may be needed when measuring position data in an absolute coordinate system. In this procedure, the reference target is used to obtain the angle orientation of the total station at the beginning of a measurement session (as for instance obtained after a calibration procedure) and checked before finishing the measurement session. Checking the position of the reference target relative to the angle orientation of the total station ensure that the total station has not moved during the measurement session.”
However, also the verification method described in the above-cited passage of WO 2009/106144 A1 has to be carried out completely manually and is, therefore, cumbersome. An again, the reliability of such a verification depends on the skill of the operating person.
More information regarding a verification of the surveying instrument's orientation during a user's measurement session in the field is not given in WO 2009/106144 A1 at all, as actually the main technical teachings of WO 2009/106144 refer to completely different subject matters like to a method for calibrating a surveying instrument (see—among many other passages—page 12, lines 7-12 of WO 2009/106144 A1) or—as a minor matter—to a method for searching for a reference target (see e.g. page 23, lines 20-27 of WO 2009/106144 A1), which both form completely different topics compared to the matter of a method for verifying a surveying instrument's external orientation during a measurement session in the field.
Therefore, there is need for an improved method to determine and verify a surveying instrument's external orientation during a measurement session, particularly wherein the method can be carried out with less effort and with high reliability —independently from the skill of the operator.